1. Field of the Invention
The present invention relates to an image processing apparatus, an image processing system, an image processing method, a program for realizing the functions of the apparatus or the system, a program for executing processing steps of the method, and a storage medium storing the programs which can be read by a computer. In particular, the present invention relates to an image processing apparatus, an image processing system, an image processing method, a program and a computer-readable storage medium, each of which is used for extracting a feature value from an object image to execute image processing of the object image such as the gradation conversion thereof and the like on the basis of the extracted feature value.
2. Related Background Art
When images photographed by photographing means such as a sensor, a camera or the like are outputted to a monitor screen, a film or the like, the gradation conversion of the images are conventionally executed to obtain the images having density values that are easy to observe on the monitor screen, the film or the like.
To put it concretely, in the gradation conversion processing, a feature value of density (hereinafter, also simply referred to as “a feature value”) is extracted from an object image, and the gradation conversion of the object image is executed by the use of the extracted feature value.
The feature value of density hereupon means a pixel value or a statistical value of pixel values in a predetermined region of an object image. The pixel value or the statistical value (the feature value) is necessary in order to convert the object image into an image having a desired distribution of the pixel values by the gradation conversion processing, especially, in order to output a predetermined image region of the object image to the film, the monitor screen or the like with a desired density or brightness distribution.
As extraction methods of the feature value, various methods have been proposed.
For example, as an extraction method of a feature value in the case where an image (an X-ray image) of a cervical spine part obtained by the X-raying thereof is outputted to a film for X-ray diagnosis, the following methods were proposed.
(1) Method Disclosed in Japanese Patent Application Laid-Open No. 2000-099708 and So Forth
The method determines a region from which a feature value is extracted (hereinafter, the region is referred to as a “feature value extraction region” or a “region of interest (ROI)”) on the basis of the contour shape of an object (a cervical spine part or the like), and then the method determines the feature value from the ROI.
(2) Method Disclosed in Japanese Patent Application Laid-Open No. 2000-163562 and So Forth
The method determines an ROI on the basis of a mean value of pixels of each line in the direction crossing an object (a cervical spine part or the like) to determine a feature value from the ROI.
However, the conventional feature value extracting methods such as the methods (1) and (2) have the following problems.
For example, in the case where an ROI is determined on the basis of the contour shape (such as positions of concavities of the contour or the like) of an object in an image of a cervical spine part in conformity with the method (1) (the method disclosed in Japanese Patent Application Laid-Open No. 2000-099708 and so forth), the ROI of X-ray images obtained by the front radiographing or the lateral radiographing of the cervical spine part could surely be determined. However, when the cervical spine is greatly bent forward at the time of cervical spine anteflexion radio graphing in which the cervical spine part is radiographed in the lateral direction thereof with the cervical spine being bent forward, there is the case where concavity of the contour of the cervical spine part in the X-ray image is not produced at the position of the neck of an object. In this case, no ROI can stably be determined.
On the other hand, in case of radiographing in conformity with the method (2) (the method disclosed in Japanese Patent Application Laid-Open No. 2000-163562 and so forth), ROI's can stably be determined in the X-ray images as long as the X-ray images were obtained by the radiographing of a cervical spine part bent forward slightly, the radiographing of a cervical spine part bent backward, and the radiographing of a cervical spine part from the front side thereof in the X-raying of cervical spine parts. However, for example, in an X-ray image 400 that is shown in FIG. 4 and has been obtained by the X-raying of a neck region that is bent forward greatly and is surrounded by a head part 401 and a shoulder part 402, a line 403 having the maximum mean value among the mean values of lines crossing the neck region crosses the jaw of the object. Consequently, no ROI can accurately be determined in such an X-ray image.